Render block embeds with full content

[avelino]

Summary

Render {{embed: ((block-uid))}} blocks with their full content (including children) instead of the current placeholder text. Block embeds are one of Roam's key features for reusing content across pages.

Roam web behavior: Block embeds render the referenced block AND all its children inline, in an indented container. The embedded content is editable in-place — edits propagate to the original block.

TUI adaptation: Render embedded blocks read-only with full content (block text + children tree). Editing the embed should navigate to the original block instead of editing in-place (TUI complexity constraint). Visual indicator (border or color) distinguishes embedded content from native blocks.

Implementation hints

Current state

• src/markdown.rs:52-62 — parse_embed_inner() extracts the block UID from {{embed: ((uid))}}
• Currently renders as: ▸ embed: block text (magenta italic) using the block_ref_cache
• Only shows the top-level block text, no children

Data fetching

• src/app.rs:84 — block_ref_cache: HashMap<String, String> stores uid → text
• Need a richer cache: uid → Block (with children) for embeds
• Add embed_cache: HashMap<String, Block> to AppState
• Fetch via pull() with a recursive selector: [:block/string :block/uid :block/order :block/open {:block/children ...}]
• Spawn async fetch same pattern as spawn_resolve_block_refs

Rendering

• src/ui/main_area.rs — add VisibleLine variants for embed content:
  • EmbedStart { depth, block_index } — visual indicator (e.g., ┌─ embed ─)
  • EmbedBlock { depth, text, block_index } — each block in the embed
  • EmbedEnd { depth, block_index } — visual indicator (e.g., └─────────)
• Render children recursively with increased depth
• Different background or border color to visually separate embed from surrounding content

Embed rendering in markdown.rs

• Instead of rendering inline, render_spans_with_refs() should return a signal that this block is an embed
• The flatten_blocks() function in main_area.rs handles the expansion

Interaction

• Embeds are read-only in the TUI
• Pressing Enter / i on an embedded block could navigate to the original block's page (if page navigation exists, issue Page navigation: follow [[links]] to open pages #1)
• Or show a status message: "Navigate to original block? (Enter to confirm)"

Edge cases

• Circular embeds (A embeds B, B embeds A) — detect and break with max depth
• {{embed: [[Page Name]]}} — page embed, not just block embed. Render page's top-level blocks.
• Missing/deleted blocks — show "Block not found" placeholder

Acceptance criteria

• {{embed: ((uid))}} renders full block content with children
• Visual border/indicator distinguishes embedded content
• Children are indented properly within the embed
• Circular embed detection prevents infinite recursion
• Missing blocks show placeholder instead of crashing
• Page embeds ({{embed: [[Page]]}}) also work